(1) Field of the Invention
The present invention relates to a digital time-division switch. More particularly, it relates to a digital time-division switch in which the time slots of a time-division transmission line and of a switching network portion are effectively utilized.
(2) Description of the Prior Art
In a general digital time-division switch, supervisory signals, voice signals, tone signals, and so forth transmitted from each line circuit portion, including subscriber circuits, interoffice trunks, a tone truck, and so on are multiplexed in a time-division manner forth, the same transmission line and are transmitted to a switching network portion including a switching network, i.e., a speech network formed, for example, of memory circuits. Control signals, transmitted from a common control unit, for controlling the line circuit portions are multiplexed in a time-division manner with voice signals transmitted from the switching network portion and are transmitted to the line circuit portion through the same transmission line. Therefore, the time slots of the time-division transmission line are assigned for other signals in addition to voice signals, i.e., supervisory signals, tone signals, control signals, and so forth. In this situation, it is preferable that the number of time slots assigned for signals other than voice signals be as small as possible in order to increase the number of voice channels per transmission line.
FIG. 1 illustrates the structure of a conventional digital time-division switch. In FIG. 1, subscriber circuits are designated by LC0, - - - , interoffice trunk circuits are designated by TK0, a tone trunk circuit for generating tone signals is designated by TNT, and a multiplexer for multiplexing a plurality of input signals is designated by MPX. All of these circuits constitute the transmitting section of the line circuit portion. D is a signal-extracting circuit, i.e., a dropper which extracts supervisory signals from the transmission line. SCN is a supervisory signal receiver which receives supervisory information, i.e., scanned information, from the subscriber circuits and the trunk circuits. SW is a switching network which operates in a time-division manner. SC is a switch controller for controlling the switching network SW. I is a signal-inserting circuit, i.e., an inserter which inserts control signals in the transmission line. SD is a supervisory data SD sender for transmitting control data, i.e., SD data to the subscriber circuits and the trunk circuits. All of these components are included in a switching network portion. PU is a common control unit or a processor for controlling the switching process. DMPX is a distributor circuit or a demultiplexer which converts time-division multiplexed signals on the transmission line into separate line signals. The distributor circuit DMPX and the aforementioned subscriber circuits LC0, - - - and interoffice trunk circuits TK0 constitute the receiving section of the line circuit portion. In FIG. 1, the transmitting section of the line circuit portion is shown on the left and the receiving section of the line circuit portion is shown on the right. However, in practice, these two sections are not separated, i.e., they are included in one circuit block constituting the line circuit portion.
In the switch of FIG. 1, a voice signal transmitted from a subscriber, for example, subscriber A connected to a subscriber circuit LC0, is converted to a digital signal, such as a pulse-code modulation signal, in the subscriber circuit LC0 and is inserted into a predetermined time slot of a transmission line as a time-division signal by the multiplexer MPX. The time-division signal is switched to another time slot corresponding to the other subscriber (not shown) by the switching network SW controlled by switch controller SC and is transmitted to the distributor circuit DMPX. The distributor circuit DMPX distributes the time-division signal to the receiving section of the subscriber circuit of the other subscriber, in which circuit the time-division signal, i.e., the digital signal, is converted to an original analog voice signal and is then transmitted to the other subscriber.
In the above-mentioned switch, the supervisory signals of the subscriber circuits and so forth, obtained by scanning the subscriber circuits, for example, on-hook or off-hook signals of each subscriber circuit, are multiplexed with the voice signals, and are transmitted on the same transmission line as that used for transmitting the voice signals. The supervisory signal is extracted by the signal-extracting circuit D and is transmitted to the common control unit PU through the supervisory signal receiver SCN. On the other hand, SD data, i.e., control data transmitted from the common control unit PU, for example, data for controlling the drive of a ringing current feeding relay, is transmitted to the signal-inserting circuit I connected to the output stage of the switching network SW through the data sender SD. The SD data is multiplexed in a time-division manner with other signals, such as voice signals, in the inserting circuit I and is transmitted to the distributor circuit DMPX, which then transmits the SD data to a subscriber circuit and so forth.
A tone signal, for example, a dial tone signal, generated by the tone trunk TNT disposed in a line circuit portion is multiplexed in a time-division manner in the multiplexer MPX and is transmitted to a subscriber telephone through the same transmission line as the voice signals and through the switching network SW, the distributor circuit DMPX, and a subscriber circuit.
FIG. 2 illustrates the assignment of multiplexed data at each point on the transmission line of the switching network portion in the switch of FIG. 1. In FIG. 2, (a) through (d) illustrate, respectively, the assignment of the data of each frame of the multiplexed signals at points a through d of the transmission line shown in FIG. 1. At point a, supervisory data (SCN) resides in time slots TS.sub.0 through TS.sub.i, and tone signals transmitted from the tone trunk TNT resides in time slots TS.sub.i+1 through TS.sub.j. At the input stage c of the switching network SW, the time slots TS.sub.0 through TS.sub.i represented by the symbol V are not used due to the supervisory data being previously extracted therefrom by the signal extracting circuit D. At the point d of the switching network SW, the time slots TS.sub.i+1 through TS.sub.j are vacant due to the tone signals therein being transferred to the time slots within the time slots TS.sub.j+1 through TS.sub.n for voice signals. At the point b, SD data is inserted into the time slots TS.sub.0 through TS.sub.i by the signal-inserting circuit I.
FIG. 3 illustrates the contents of data stored in a speech path memory comprising a switching network SW and in a control memory CM provided in a switch controller SC. In FIG. 3, (a) and (b) illustrate the contents of a frame of data in another manner at points a and b of the transmission line in FIG. 1, respectively, and correspond to (a) and (b) of FIG. 2, respectively.
In FIG. 3, the contents of data stored in the speech path memory corresponds to the contents of data at the input stage c of the switching network SW which is shown in (c) of FIG. 2. The speech path memory has addresses TS.sub.0 through TS.sub.n corresponding to the time slots TS.sub.0 through TS.sub.n. In FIG. 3, the addresses of the speech path memory are designated by the same symbols as those of the corresponding incoming time slot numbers. In the addresses TS.sub.0 through TS.sub.i of the speech path memory, no significant data is stored, and in the addresses TS.sub.i+1 through TS.sub.j, tone signal data is stored. In the addresses TS.sub.j+1 through TS.sub.n, voice signal data is stored. For example, in the address TS.sub.i+1 corresponding to the time slots TS.sub.i+1, dial tone (DT) data transmitted from the tone trunk is stored, and in the address TS.sub.j+1 corresponding to the time slot TS.sub.j+1, voice signal data from the subscriber A is stored.
Each address of the control memory stores a time slot number or destination address. The control memory effects a switching operation by sequentially reading out the data (destination address) stored in the control memory from the first address to the last address thereof and by inserting the data stored in the corresponding sequential addresses of the speech path memory, which destination addresses are designated by the content stored in each address of the control memory, into the outgoing time slots corresponding to the destination addresses in the control memory. For example, assume that the time slot number TS.sub.j+1 is stored in the address T.sub.i+1 of the control memory and that another time slot number TS.sub.i+1 is stored in the address T.sub.j+1 of the control memory. In this condition, at the output stage of the switching network, voice signal data, which was originally in incoming time slot TS.sub.j+1, transmitted from the subscriber A is inserted into the outgoing time slot TS.sub.i+1, and dial tone signal data, which was originally in incoming time slot TS.sub.i+1, transmitted from the tone trunk TNT is inserted into the time slot TS.sub. j+1. Therefore, at point b of the transmission line, the voice signal of the subscriber A and the dial tone signal data transmitted from the tone trunk TNT are assigned in the time slots TS.sub.i+1 and TS.sub.j+1, respectively, and, moreover, SD data is inserted into the time slots TS.sub.0 through TS.sub.i by the signal-inserting circuit I, as is shown in (b) of FIG. 3. As a result, the dial tone signal data inserted into the time slot TS.sub.j+1 is transmitted to the receiving section of the line circuit of the subscriber A.
In the above-mentioned conventional switch, since the time slots allocated for the supervisory data and for the control signals are vacant inside the switching network SW, they are inefficiently utilized.
Further, in the above-mentioned conventional switch, tone signals which are transmitted from the switch to each subscriber, such as a dial tone signal or a ring back tone signal, are inserted into the time slots for voice signals transmitted from the transmitting section of the line circuit portion. Since such tone signals are only transmitted one way and do not necessitate time slots for receiving signals from the switching network SW, the time slots for receiving signals from the switching network SW are inefficiently utilized.
As is diagrammatically illustrated in FIG. 4, the switching network SW of a time-division switch generally comprises transmitting terminals, i.e., time slots for transmitting signals (TM1, TM3, etc.), and receiving terminals, i.e., time slots for receiving signals (TM2, TM4, etc.). In the case of voice signals, both the transmitting terminal and the receiving terminal, for example, TM1 and TM2, are used. However, in the case of tone signals, a tone signal transmitted from tone generator TNG is input into the transmitting terminal, for example, TM3, and the receiving terminal, for example, TM4, corresponding to the transmitting terminal TM3 is not used. Therefore efficient utilization of the time slots is not realized.
Moreover, in the above-mentioned conventional switch, since tone signals are transmitted via part of the time slots which could be used for voice signals, the number of time slots which are actually used for transmitting the voice signals is thereby relatively decreased. Therefore, the number of subscribers connected to a line circuit portion is small, and the degree of multiplex of the time-division switch is relatively decreased.